The goals of this project are to understand how spinal circuits are used to coordinate normal movement in humans and how malfunctioning of spinal circuits contributes to abnormal movements in neurological disorders. We have particularly focused on neurological disorders producing excessive muscle contraction or spasticity. During FY2002, our efforts to define clinical subtypes of Primary Lateral Sclerosis, a degenerative disorder of corticospinal neurons, expanded to include MRSI to assess motor cortex integrity. Physiological studies to determine how corticospinal loss alters the excitability and firing behavior of motor neurons in this disorder continue, and preliminary work to assess interventions to induce plasticity in spinal circuits was initiated.